The present invention relates to an abrading plate and a method for polishing an object such as semiconductor wafer using the abrading plate to obtain a flat and mirror finish.
Recently, as the density of circuit integration in semiconductor devices becomes ever higher, circuit patterns are becoming finer and interline spacing narrower. Especially since the line width becomes finer, the depth of focus of the stepper becomes very shallow in photolithographic reproduction of circuit patterns, and the surface of the wafer placed at the focal plane of the stepper must be flat to produce the required degree of image sharpness. A method of obtaining a flat surface is to polish the object in a polishing apparatus.
A type of conventional polishing apparatus comprises a turntable having a polishing cloth and a top ring which is pressed against the turntable with a given pressure while the polishing object is held therebetween, and supplying a polishing solution to the interface to produce a flat and mirror polished surface. This approach is called chemical mechanical polishing (CMP).
FIG. 1 shows essential parts of an example of a conventional CMP apparatus. The apparatus is provided with a rotating turntable 5 having a polishing cloth 6 such as a urethane cloth; a top ring 1 holding an object 4 such as a semiconductor wafer 4 against the cloth 6; and a spray nozzle 9 for supplying a polishing solution Q to the polishing cloth 6. The top ring 1 is connected to a top ring shaft 8, and the top ring 1 holds a semiconductor wafer 4 in contact with an elastic mat 2 such as polyurethane. The top ring 1 has a cylindrical guide ring 3 on its outer periphery so that the wafer 4 would not be disengaged from the bottom of the top ring 1. The guide ring 3 is fixed to the top ring 1, and the wafer 4 is held within the holding surface so that the wafer 4 would not jump outside of the top ring due to frictional forces with the cloth 6. The top ring 1 is supported on a spherical bearing 7 so that it can be tilted with respect to the shaft 8.
While holding the wafer 4 in the bottom surface of the elastic mat 2 of the top ring 1, wafer 4 is pressed against the cloth 6 on the turntable 5, and the turntable 5 and the top ring 1 are independently rotated so as to slide the surface of the wafer 4 relative to the cloth 6 to polish the wafer 4. In this case, a polishing solution Q is supplied from the nozzle 9 to the top surface of the cloth 6. The polishing solution comprises abrasive particles, for example, such as silica particles, suspended in an alkaline solution, which has two effects of CMP for a semiconductor wafer, chemical polishing using an alkaline solution, for example, and mechanical polishing using abrasive particles.
However, the conventional polishing methods of CMP using such a slurry solution containing numerous abrasive particles presents two operational problems.
The first problem is that, during the initial stage of polishing, raised regions of the surface structure are preferentially removed, but gradually, depressed regions are also removed. Therefore surface irregularities are difficult to decrease. It is considered that this phenomenon in CMP is created because a relatively soft cloth is used along with freed abrasive particles contained in the polishing solution, so that not only the raised portions, but also the depressed portion of the surface structure, are removed by such abrasive particles. FIG. 2 illustrates such problems of the conventional CMP, which shows irregularities caused by a raised portion and depressed portion of surface film thickness structure on the vertical axis and relative time on the horizontal axis. As indicated in this graph, at a relative time of 1 to reach a stage of surface removal, the raised regions are polished from a height of about 27,000 angstroms to a height of 16,000 angstroms, but the depressed regions are also polished from 20,000 angstroms to 16,000 angstroms, at which point the irregularities are eliminated. FIG. 3A shows surfaces profiles of a fine surface structure in an initial stage, FIG. 3B in a middle stage, and FIG. 3C in a final stage of polishing. As illustrated in these drawings, the irregularities are very difficult to be removed, and consequently, it is a time-consuming operation.
The second problem relates to cost and environmental considerations. The polishing solution is usually a slurry containing a fine silica powder in suspension, but to obtain a uniformly flat surface of high quality, it is necessary to supply the solution in a liberal quantity to the polishing surface. However, most of the solution supplied is actually discharged as a waste solution without contributing much to the polishing process. Polishing solutions used for high precision polishing of semiconductor devices are expensive, which is a factor leading to a problem of high polishing cost. Also, because such polishing solution in a slurry condition contains a large quantity of abrasive particles such as silica particles, the maintenance of the working environment is apt to be difficult. That is, contamination of the solution supply system and drainage system by the slurry is serious, and the waste solution must also be treated extensively before discarding it. Also, after a CMP process, the wafers are washed to remove the polishing solution, but the waste water from this operation also must be treated in a similar manner and poses an environmental problem.
To solve the above described problems, it is therefore an object of the present invention to provide an abrading plate and a method using the abrading plate for preferentially removing only the raised regions in a patterned semiconductor wafer having fine irregularities fabricated on the surface, with, when the irregularities are eliminated, the abrading plate having a self-stopping function to stop polishing automatically.
Another object of the present invention is to provide a polishing method and an apparatus using the abrading plate which easily enables additional polishing to remove a certain film thickness uniformly after a height difference is eliminated by polishing.
To achieve the objects of the present invention, there is provided an abrading plate which comprises abrasive particles having a chemical purity of not less than 90% (preferably higher than 94%) and a particle size of not more than 2 xcexcm (preferably less than 0.5 xcexcm); a binder material; and a given volume of porosity, wherein a ratio of the abrasive particles and the binder material is 1:x, where x is not less than 0.5 by volume (the binder material per 1 unit of the abrasive particles is not less than 0.5 unit), and proportions of abrasive particles, a binder material and porosity are, respectively, not less than 10%, not more than 60% and 10xcx9c40% by volume.
According to the present invention, the abrading plate thus produced has an optimized composition of particles, binder and porosity by volume, so that raised regions of the polishing surface is preferentially removed from the object surface and depressed regions are not removed. Therefore, after the raised regions are flattened and the surface has become level, continued polishing will not proceed to change the film thickness of the surface structure. Stopping or removal rate reduction is achieved automatically, and is termed a self-stopping capability. If the abrasive particles are in excess or the binder material is insufficient, abrasive particles are easily self-generated (released), so that abrading continues even after a level surface has been produced, and the self-stopping function cannot operate. If the amount of abrasive particles is insufficient or binder material is in excess, abrasive particles are difficult to be self-generated (released) and the polishing rate is reduced so that the polishing capacity is decreased. A similar tendency applies to porosity, and it is preferred to have a porosity 10xcx9c40% by volume or more, preferably 15xcx9c30% by volume, to give the self-stopping capability to the abrading plate. In other words, too much porosity makes the abrading plate too soft and promotes generation of released particles, while too little porosity makes the abrading plate too hard to discourage self-generation of freed particles (released). Because the particles are less than 2 xcexcm in size, the chances of scratching the wafer are reduced, but it is preferable to use particle sizes of less than 0.5 xcexcm.
In general, the polishing rate is increased by having a large number of released abrasive particles in the sliding interface. When there are a lesser number of freed abrasive particles, the polishing rate is reduced, and a wafer processing ability, namely throughput, is reduced. Accordingly, when the surface of the semiconductor device patterned wafer having raised and depressed regions is polished by the abrading plate, a high surface pressure is applied to the raised region of the wafer by the abrading surface of the abrading plate, thus the raised portion bites and shaves the abrading surface of the abrading plate to release the abrasive particles through an initial stage to a final stage of the polishing. A large number of abrasive particles are released and produced during the polishing stage, and thus polishing proceeds at a relatively large polishing rate by the released abrasive particles. Therefore, in the final stage of polishing, when the surface has become level, the wafer has few raised regions left on the wafer to bite into the abrading plate so that a lesser number of released particles are produced from the abrading plate. During polishing, the sliding interface shifts its location constantly, and the residual released abrasive particles are lost from the sliding interface. Thus, the amount of the released abrasive particles remaining on the sliding interface becomes extremely small, and polishing action stops eventually to thereby provide a self-stopping function, which occurs as an extreme decrease of the polishing rate.
In the abrading plate, proportions of abrasive particles, a binder material and porosity should be 10xcx9c60% (preferably 20xcx9c50%), 30xcx9c60% (preferably 35xcx9c55%), and 10xcx9c40% (preferably 15xcx9c30%) by volume, respectively, to achieve a self-stopping capability as described above.
It is preferable that an abrading plate is comprised of abrasive particles having a chemical purity of not less than 90% (preferably higher than 94%); and it is preferable that the abrading plate be comprised of such particles, a binder material and porosity. Accordingly, the abrading plate made by such abrasive particles that can be obtained easily will present little danger of contaminating device wafers (patterned wafers).
A method is presented for polishing a semiconductor device wafer, having fine surface structure fabricated on a polishing surface, using an abrading plate. The method includes the conditioning steps of: dressing an abrading surface of the abrading plate so as to produce a roughened structure on the abrading surface; removing released particles attaching to the roughened structure; and polishing the polishing surface with a conditioned abrading plate having a stabilized polishing rate generated by the foregoing steps. According to the method, the characteristic feature of the self-stopping function of the present abrading plate can be fully effective to be utilized in polishing the semiconductor device wafer.
Also, the present method of polishing of a semiconductor device wafer having fine structures fabricated on its surface includes a feature that an abrading surface of an abrading plate is dressed to produce a micro-rough surface, and released abrasive particles attached to the abrading surface are removed before pressing onto a surface of an object to be polished, so that the abrading surface has reached a stabilized condition to polish a blanket wafer (a wafer covered by an overall film) at a sufficiently low polishing rate. This procedure is effective in generating the self-stopping capability.
Also, the present method of polishing of a semiconductor device wafer having fine structures fabricated on its surface includes a feature that additional surface removal is performed with an abrading plate using a liquid not containing any abrasive particles for a specific duration, followed by additional polishing using a slurry containing abrasive particles to the surface to be polished. A specific duration is a duration sufficient for the surface to be leveled by removing the high and low spots. According to this method, it enables the use of the same abrading plate to obtain additional surface removal by using a slurry containing a large amount of released abrasive particles. Because this polishing is carried out using a slurry containing a large amount of abrasive particles, surface removal can be achieved in a relatively short time to obtain a desired film thickness.
Also, the present method of polishing of a semiconductor device wafer having fine structures fabricated on the surface includes a feature that the additional surface removal is performed using an abrading plate and a liquid not containing any abrasive particles for a specific duration, followed by additional polishing using released abrasive particles being produced from concurrent dressing of the abrading plate. This method enables the use of the same abrading plate so that the released abrasive particles, being produced by carrying out dressing concurrently with the polishing by the abrading plate, are used to obtain quick removal of the surface material, thereby raising the polishing rate. Therefore, it is not necessary to have an additional facility such as the polishing solution supply device so that a regular facility based on the abrading plate can be used to perform additional surface removal.
Also, the present method of polishing of a semiconductor device wafer having fine structures fabricated on its surface includes a feature that additional removal is performed using an abrading plate and a liquid not containing any abrasive particles for a specific duration, followed by additional polishing using a polishing cloth and a slurry containing abrasive particles. This method enables the use of an existing facility to carry out additional surface removal based on a conventional slurry and polishing cloth.